1. Field of the Invention
The present invention generally relates to optical pickup devices, and more particularly to an optical pickup device having a lens for projecting a light onto a recording medium and optically reading information stored in the recording medium by controlling the orientation of the lens.
2. Description of the Related Art
In a disk unit loaded with a disk recording medium (hereinafter referred to as a disk), such as a CD-ROM apparatus, for instance, information stored in the disk is read by projecting a laser beam onto the disk. An optical pickup device incorporated into this type of disk unit has an objective lens for projecting the laser beam emitted from a laser diode onto the disk, and controls the orientation of the objective lens so that the laser beam projected through the objective lens is focused on a recording surface of the disk.
That is, in the optical pickup device, focus control and tracking control are performed so that the objective lens follows the surface fluctuation of the disk or the motion of a track due to eccentricity. Such control of the objective lens is performed by an actuator utilizing electromagnetic force. Generally, the actuator is composed of a combination of a coil and a magnet.
A lens holder holding the objective lens is reduced in size and weight so that focus control and tracking control can be performed easily. Further, the lens holder is supported by a support structure where four wire-like elastic supporters are provided in parallel so that a sensitive control operation of the objective lens is performed by a driving force from the actuator.
However, the conventional optical pickup device has its movable part such as the lens holder moved easily in order to increase the accuracy of the above-mentioned focus and tracking controls. Therefore, variations in component size, for instance, cause a deviation between the center of gravity of the movable part and the driving point position of the actuator, thus generating a turning moment in the movable part. This turning moment causes unnecessary vibrations to be generated in the movable part, thus destabilizing focus and tracking controls.
Further, insufficient stiffness of the movable part due to a thinner lens holder increases the magnitude of higher-order-harmonic resonance so that a servo system starts oscillating.
Moreover, if variations in component size or assembly errors are caused, the center of gravity of the movable part or the position of a driving point on which the driving force from the actuator is exerted is deviated. Therefore, unnecessary vibrations may be caused by a turning moment generated in the movable part.
In the optical pickup device, a natural frequency may be considered to be a factor of the generation of unnecessary vibrations. Since the natural frequency is determined by a component shape and/or material, it is required to increase or decrease the level of the natural frequency by changing such a factor (factors).
Further, it is also possible to eliminate the deviation of the center of gravity of the movable part or the driving point of the actuator by increasing the processing or assembly accuracy of components. However, increasing the processing or assembly accuracy of components causes a problem of an increase in the costs of mass production.
Furthermore, in order to manage the position of the center of gravity with high accuracy, it is required to be informed quantitatively of an error between calculated and actual positions of the center of gravity by repeating trial production and evaluation. Therefore, in the case of managing the position of the center of gravity, there has been a problem that a product development period becomes longer due to repetition of the trial production and evaluation.
In addition, in order to lower the level of the natural frequency of the movable part, it is required to increase the stiffness of an individual component. However, it has been difficult to increase component stiffness by increasing component thickness due to restriction of component interference. It is possible as a method of solving such a problem to increase stiffness by forming the lens holder with a metal strip being inserted therein by insert molding. However, this causes a problem of an increase in production costs.